as a result, security has stepped up in the united states, the united kingdom and the middle east. >> nobody wants to be the person who overlooked a clue. that's happened in the past. and we've paid for it. >> reporter: u.s. officials say there's no credible information of an imminent attack. but white house counterterrorism official, john brennan, the the al qaeda in yemen is a significant threat. >> they continue to seek the ability to strike our homeland. >> reporter: one year ago, president obama ordered the killing of osama bin laden. yesterday, he called on americans to remember those who participated in the operation. >> the american people rightly, remember what we, as a country, accomplished. in bringing to justice, somebody who killed over 3,000 of our citizens. >>> u.s. authorities say they made adjustments to airport screenings, such as turning up the radiation used to try to detect body bombs. rob? sunny? >> sobering news. tahman bradley in washington. thanks very much, tahman. >>> bin laden's death is figuring prominently in the presidential campaign. mitt romney is now